1. Field of the Invention
The present invention relates to a printed wiring board unit for use with an electronic apparatus, in which measures are taken against unwanted radiation noise by mounting a second printed wiring board comprising a digital circuit onto a first printed wiring board which constitutes a power circuit.
2. Description of the Related Art
In general, an electronic apparatus comprises a power circuit section formed of mainly an analog circuit, and a control circuit section formed of mainly a digital circuit. The power circuit section is formed of large lead-type electronic parts which are circuits themselves, such as an AC/DC converter or a DC/DC converter. The control circuit section is formed of small surface-mounting-type electronic parts.
In recent years, electronic apparatuses have been getting faster, and the clock frequency in digital circuits is becoming increasingly higher. In response to this, countermeasures against unwanted radiation noise, which might cause a malfunction of other electronic apparatuses, are becoming difficult.
For this reason, conventionally, the digital circuit is formed on a printed wiring board separate from a power circuit, and power is supplied and signals are transmitted by using cables. FIG. 6 shows a top plan view of a conventional example in which a digital circuit and a power circuit are formed in separate printed wiring boards. In FIG. 6, a digital circuit is formed on a two-layer printed wiring board 9, and an IC, which is a CPU 3 operating at the highest internal clock frequency in the unit, a gate array IC 4 which controls the peripheral circuits of the CPU 3, and an IC of a dynamic memory 5 are mounted thereon. A printed wiring board 1 forming mainly a power circuit is normally made of a single-sided board, and 5-V power is supplied to the printed wiring board 9 through a connector 10 and a cable 11. Also, some digital signals are transmitted from the printed wiring board 9 to signal patterns of the printed wiring board 1 through the connector 10 and the cable 11. Bundled wires are also often used instead of the cable 11.
In the case of a mounting construction in which a power circuit board and a digital circuit board are separated, and power is supplied from the power circuit board to the digital circuit board through a cable in the manner described above, noise generated in the digital board is also transmitted to the cable and the cable becomes an antenna, generating high levels of unwanted radiation noise.
In contrast, an example in which a control circuit including a digital circuit and a power circuit are formed on the same printed wiring board is disclosed in Japanese Patent Laid-Open No. 6-314000. FIG. 7 shows a top plan view of a conventional example in which a digital circuit and a power circuit are formed on the same printed wiring board. With such a construction, since cables or bundled wires are not used, unwanted radiation noise can be considerably reduced to less than that in the former case.
In a normal case, a power circuit uses many large parts and since the pattern density may be low, an inexpensive board such as a single-sided board is used. However, in the case where a digital circuit pattern is formed on such a board, since it is difficult to form a digital ground which serves as a return path of a digital signal along the vicinity of a signal line with the shortest spacing, unwanted radiation noise is increased.
Therefore, no problem is posed when the number of digital signal lines is relatively small, a space is allowed, a ground can be appropriately wired, and the clock frequency is low. However, if the digital circuit section is complex and the number of wiring patterns become very large, it is difficult to wire a ground pattern. Furthermore, if the clock frequency is high, and because grounding is inherently more poor than that of two-layer and multi-layer boards, unwanted radiation noise characteristics become sharply deteriorated.
In order to prevent this, the single-sided board is replaced with a two-layer board or a multi-layer board, resulting in greatly increased costs. However, the power circuit part, which occupies most of the board, does not require such an expensive board.
In recent years, in digital circuits, the number of lines of the buses that the CPU processes has increased sharply. In response to this, the number of resistance parts which are inserted into the data bus and the address bus has also increased sharply. For example, the number of data bus lines which are connected to the CPU has increased from 8 to 16 to 32, and recently, 64 data bus lines are beginning to be most common. Various LSIs and memories, which operate at high speeds, are connected to such a CPU in order to control circuits peripheral to the CPU. Formation of such a complex digital circuit, a power circuit and an analog circuit, of which the latter two require only relatively simple patterns, causes either an increase in unwanted radiation noise or a greatly increased cost.
An object of the present invention, which has been achieved in view of the above-described circumstances, is to provide a printed wiring board unit for use with an electronic apparatus, in which a digital circuit board is mounted in a layered structure onto a power circuit board and unwanted radiation noise characteristics are improved without increasing the cost.
Another object of the present invention is to form a digital circuit board at a high density and a small size so as to mount it like other electronic components on a power circuit board.
The above and further objects, aspects and novel features of the invention will become more apparent from the following detailed description when read in connection with the accompanying drawings.